Project Summary/Abstract Project 1 Chronic GVHD (cGVHD) is the most important adverse long-term consequence of allogeneic hematopoietic cell transplantation (HCT). Responsible for the majority of mortality occurring after 1-2 years from HCT, cGVHD is also an important cause of chronic morbidity and diminished quality of life after HCT. The overarching theme of this proposal is to study the role of the B cell in the pathophysiology of cGVHD, and Project 1 is dedicated to clinical trials that specifically target the B cell compartment for the prevention and treatment of cGVHD. In Specific Aim 1, we target preclinical cGVHD with a prophylactic strategy aiming to deplete B cells as they reconstitute early after HCT. In a phase II study conducted in the prior funding period, we addressed the hypothesis that the early post-HCT administration of the monoclonal antibody, rituximab, would reduce the incidence and severity of cGVHD, allowing markedly less systemic corticosteroid use. While effective, prevention was incomplete, so we now plan a multicenter, randomized phase II trial of B cell depletion as a preventative strategy against cGVHD using a novel second generation antibody. Recognizing that B cells alone cannot be solely responsible for the occurrence of cGVHD, we combine the strategy of B cell depletion using rituximab with regulatory T cell (TREG) enhancement using low-dose IL-2 in Specific Aim 2. In the prior funding period, we were able to demonstrate that IL-2 was able to increase the number of circulating TREG cells and this was correlated with clinical response in established steroid-refractory cGVHD. In this project we explore the utility of this two-pronged approach in patients with newly diagnosed cGVHD in an attempt to more effectively treat cGVHD and prevent the long-term sequelae of cGVHD. Specific Aim 3 will examine a series of promising compounds for the treatment of advanced cGVHD, all of which target B cells, either through the B cell receptor or through related signaling pathways. In the later years of the grant compounds with activity will be applied earlier in the development of cGVHD. The clinical studies we propose in Project 1 are intimately interlocked with the other Projects and Cores in this Program. In Project 1 we extend the basic biological discoveries of Project 3 and pre-clinical studies of Project 2 into the clinical setting.